Toll-like receptors (TLRs) play a critical role in innate immune recognition and the control of adaptive immune responses to infection. The mechanisms responsible for the initiation of autoimmune responses, however, remain largely unknown. The goal of this proposal is to investigate the possible role of TLR recognition and signaling pathways in the etiology of autoimmune disease, focusing on IDDM as a model autoimmune disorder. The first specific aim of this proposal is to investigate the consequences of the genetic blockade of TLR function on the development and progression of IDDM in NOD mice. The block of TLR function will be achieved by generating TLR2-, TLR4- and MyD88- deficient mice, as well as MyD88 dominant-negative transgenic mice, on the NOD background. This specific aim will also address the mechanisms of the protective effect of TLR agonists, such as Complete Freund's Adjuvant, on the development of diabetes in NOD mice. The second aim of the proposal is to investigate the consequences of TLR stimulation on diabetes development in mice that are engineered to express TLR ligand endogenously in pancreatic a cells. In these mice, TLRs will be activated constitutively and, importantly, in the absence of infection. The outcome of TLR activation under these conditions will be tested in both diabetes-prone NOD mice, as well as in mouse strains that do not develop IDDM spontaneously. The results of these studies should help to address a fundamental issue - the role of the Toll pathway in the mechanism of an autoimmune disease.